There are many different optical switches presently proposed. The attractions of using all-optical switches are significant. All-optical switches steer light pulses among different fiber spans without converting them into electrical signals. They promise to relieve potential capacity bottlenecks, reduce costs, and make it easier for operators of telecommunications system to deploy future developments in transmission technology.
But reaping these rewards means taking some big risks. In particular, the switching fabric that will form the basis of most all-optical switches is at an early stage of development They include arrays of tiny tilting mirrors, liquid crystals, bubbles, holograms, and thermo and acousto-optics. At present, however, none of them are close to being ready for widespread deployment in carrier networks.
MEMS (microelectro-mechanical systems) based switches use minuscule mechanisms sculpted from semiconductor materials such as silicon. They're already in widespread use in other industries and are starting to be used in components for telecommunications equipment.
In the field of optical switches, MEMS switches are used in a variety of ways. These include arrays of tiny tilting mirrors, which are either two-dimensional (“2D”) or three-dimensional (“3D”).
In a typical 2D array, the mirrors simply flap up and down in the optical equivalent of a crossbar switch. When they're down, light beams pass straight over them. When they're up, they deflect the beam to a different output port. 3D subsystems can support thousands of ports in theory, but this hasn't been proven in practice. Their switching speed is relatively slow. It is under 10 milliseconds for 4×4 and 8×8 switches. But the switching time increases to 20 ms for the larger 16×16 devices.
The presence of moving parts raises questions about mechanisms sticking, wearing out, or being damaged by vibration. Losses increase substantially if multiple subsystems have to be linked together.
With holography-based switches, an electrically energised Bragg grating (a series of stripes of different refractive index materials, each of which reflects a specific wavelength of light) is created in the form of a hologram inside a crystal. When voltage is applied, the Bragg grating deflects the light to the output port. With no voltage, the light passes straight through. Each input fiber requires a row of crystals, one for each wavelength on the fiber. Such switches have high scalability and are suitable for switches with many thousands of ports; have very fast switching speeds; and can switch from one wavelength to another in a few nanoseconds. They are potentially reliable as they have no moving parts; have low losses, and have good port-to-port repeatability as there is no path dependency within the switch.
However, high voltages are required, placing demands on the electronic supply equipment. Also, they can't compete with MEMS switches when handling groups of wavelengths being switched together from one fiber to another.
Thermo-optical technology is used for making small optical switches—typically in the 1×1, 1×2, and 2×2 range. It's a planar technology, so larger switches can be formed by integrating basic 1×2 components on the same wafer.
There are two basic types of thermo-optic switch: a digital optical switch (DOS) and interferometric switches. Interferometric switches have the advantage of being more compact, but are wavelength sensitive. For this reason they usually require some form of temperature control.
Size is limited not by optical losses, but by the power consumed in switching. Therefore, scalability is high. But the switching speed depends on how fast it is possible to heat the material. Polymer switches typically achieve a switching speed of a few milliseconds. Silica is usually slower, at around 6 to 8 ms. As there are no moving parts they are potentially reliable. However, repeated heating and cooling may limit the life of switches. Silica has very low losses, but polymer losses are higher.
Port-to-port repeatability in good, and polymer-based switches require very low switching power, typically 5 milliwatts. Silica switches consume about 100 times more power.
The progress in the development of optical computers is severely hindered due to the lack of appropriate materials to design the fast responding photonic switches, which can be operated with the help of light beams. The field of biomolecular electronics is currently focusing on finding the remedy to this problem through the adoption of suitable biological materials for this purpose.